The present invention relates to an air cleaning apparatus, especially an apparatus for cleaning of room air. Room air air in dwelling rooms, office or other work-rooms, meeting rooms, etc., or in other words, rooms in the ordinary sense of the word. Room air also refers to air in other spaces, such as a space defined by a ventilation duct, which is separated from the open air and thus holds a well-defined body of air.
More particularly, the invention relates to an air cleaning apparatus which comprises an ionizing device having a unipolar ion source formed by a corona discharge electrode, and an electrostatic precipitator connected to a high-voltage source and having a flow-through passageway for the air to be cleaned and two groups of electrode elements disposed in the flow-through passageway, the electrode elements of one group being interleaved with and spaced from the electrode elements of the other group and arranged to be at a potential different from that of the said other group.
Today, air cleaning apparatus of this kind exist in the form of two-stage electrostatic filters in which the ionizing device comprises a corona discharge electrode in an ionizing chamber at the upstream or inlet side of the precipitator. The walls of the ionizing chamber enclose a well-defined space in which the charging of the dust particles takes place more or less simultaneously with the entry of the air into the precipitator. This space may be considered as an upstream extension of the flow-through passageway of the precipitator, because all air that passes through the ionizing chamber and flows past the corona discharge electrode, flows on through the precipitator.
The efficiency of air cleaning apparatus of this kind is dependent on the efficiency of the charging of the dust particles which the airflow through the ionizing chamber carries and which are to be separated from the air in the precipitator.
One way of achieving an efficient charging of the dust particles is to pass a strong corona through the corona discharge electrode, but a strong corona current has the undesirable effect of causing a substantial generation of ozone at the corona discharge electrode.
It is also possible to achieve an efficient charging of the dust particles with a weak corona current by designing the ionization chamber such that the dimension of the space charge region which is defined by the ionizing chamber and through which the airborne particles pass during their travel to the precipitator is large in the direction of flow so that the particles will have a long dwelling time in this region and, consequently, the time available for charging of the particles will also be long. WO93/16807 discloses a two-stage electrostatic filter in which this technique of achieving an efficient charging of the dust particles is embodied. Using the two stage electrostatic filter described in this publication it is possible to achieve a dust separation which is greatly improved over that of other prior art two-stage electrostatic filters, even though the corona current is very weak.